In vehicle behavior control techniques such as VSC (Vehicle Stability Control), various controls are performed by using a detected value of a steering angle sensor mounted on a steering wheel. When a steering angle sensor of a type that outputs a relative angle is used, the zero point (neutral position) of the steering angle sensor is first detected, and calculates a steering absolute angle based on the detected zero point (see, for example, Patent Document 1). Accordingly, it is important to accurately detect the steering angle zero point.